The generation and recording of borehole acoustic waves is a key measurement employed in oilfield borehole logging. Many borehole tools and methods are currently available for taking acoustic measurements. Some tools include a single source of sonic waves and two or more receivers, however, most of the tools now include two or more acoustic sources and many receivers arranged in an array. While the currently available acoustic tools are useful in providing a large range of information regarding the surrounding formation and the borehole parameters, a primary use of acoustic borehole measurements is the estimation of compressional wave and shear wave formation slowness.
Compressional wave formation slowness is typically estimated using travel times acquired via a first motion detection process. In the case of a single source, two receiver tool suggested by the prior art, formation slowness is estimated by subtracting the arrival times between two receivers and dividing by the inter-receiver spacing. This estimate, however, is subject to inaccuracies due to tool tilt, borehole washouts, bed boundary effects, etc. Additional acoustic sources and receivers and more robust methods such as STC (Slowness-Time-Coherency analysis) and semblance processing, among others, have been used to reduce the inaccuracies introduced by such environmental effects.
Semblance processing can produce very accurate slowness values for a subterranean formation, but it is quite expensive. Semblance processing generally requires the acquisition (and, if processing takes place at the well site, communication uphole) of “long” waveforms. The speed of a logging operation is often hindered by the time it takes to gather long waveform segments and telemeter all the data from the downhole position to surface equipment.
While semblance processing provides reliable slowness estimates, it requires significant computer resources and the acquisition of long waveforms. Many well developers, however, are primarily interested in compressional slowness values. Therefore, recording just the first break of the compressional wave could be enough to obtain compressional slowness very quickly. The first arrival signals may then be used to generate formation slowness estimates. However, current methods of detecting first arrival signals of interest do not result in useful results without a skilled operator monitoring and changing dozens of parameters as the logging tool traverses a borehole. The necessity of changing parameters is typically a result of variations in the formation. Highly skilled operators with the expertise necessary to adjust the logging collection parameters and processing techniques are very expensive. Many logging applications would benefit from a reliable, less expensive logging technique, especially a logging method that reduces or eliminates the number of parameters that must be adjusted during a logging operation.
The present invention is directed to overcoming, or at least reducing the effects of, one or more of the problems outlined above.